Memory Support for more than 32GB for the Host OS previously required Windows Server 2008 Enterprise or Datacenter Editions. Even though Windows Server 2008 R2 Standard only supports 32GB, the free Microsoft Hyper-V Server 2008 R2 supports greater than 32GB. Very Cool!
Also noteworthy, similar to the Enterprise and Datacenter Editions, the free Microsoft Hyper-V Server 2008 R2 includes: Host Clustering, Live Migration and Support for greater than 4 Physical Processors (Host OS).
Check out the table on Microsoft's Hyper-V Server page.
